It is known that the water content of a raw material gas used in the production of a semiconductor greatly affects the device characteristics. In particular, for the production of a GaN crystal, which is one of the blue light emitting devices, ammonia having a low water concentration is necessary. In order to measure the water concentration in ammonia, various methods have heretofore been proposed. Particularly for measuring water content in a low concentration in ammonia, (1) gas chromatography (GC method), (2) thermal decomposition dew point method, (3) laser spectrometry and (4) infrared spectrometry are known.
JP-A-9-142833 (the term JP-A as used herein means “unexamined published Japanese patent application”) discloses a method of reacting water in ammonia with calcium carbide as a reacting agent and detecting the acetylene generated by GC method. This method has a problem in that not only the water content but also organic impurities in the calcium carbide reacted are detected and to avoid this, high-purity and stabilized calcium carbide is necessary but this is hardly available at present. In addition, the GC method is low in the accuracy and not practical for the analysis in the level of several ppm or less because the system has a switch cock for feeding a sample or a backflash cock for removing ammonia before it enters the detector, and the water adsorption onto the inner surface of pipelines increases.
SEMI Standard (SEMI C3.12-94) and JP-A-8-201370 describe a thermal decomposition dew point method. The thermal decomposition dew point method is a method of decomposing ammonia into nitrogen and hydrogen at a high temperature near 1,000° C. using a Ni catalyst or a noble metal catalyst, and measuring the water content by a dew-point instrument. According to this method, oxygen in the gas reacts with hydrogen to produce water and the water content could be determined in excessive values. Oxygen is contained not only in the raw material gas but also as an oxide of the catalyst or pipeline material exposed to high temperatures, which may also cause hydrogen reduction to produce water. Therefore, the reliability for accuracy decreases in the measurement of water content of 1 ppm or less. Moreover, use of flammable ammonia in a high temperature environment of 1,000° C. or more is dangerous and safety equipment in a large scale is necessary. Thus, this method is not simple and convenient.
Laser spectrometry is described in Proceedings of the 5th International Symposium on Semiconductor Manufacturing (1996. jointly sponsored by UCS/IEEE/SEMI), page 321. In measuring the water absorption in the near infrared region by laser spectrometry, since water is present in the vicinity of ammonia in the absorption region, a wavelength resolving power of high level is necessary. However, at present the separation cannot be attained due to the interaction between gas molecules. Therefore, ammonia gas having a decreased water content as low as negligible must be used as a reference gas. However, it has been heretofore difficult to simply and conveniently prepare high-purity ammonia which has a decreased water content and can be used as the reference gas.
The infrared spectrometry has a problem in that the ammonia has a broad absorption band and particularly when a water concentration of 100 ppm or less is measured, the absorptions by ammonia and the absorptions by water cannot be easily separated even if the former are weak because such an absorption by ammonia is present near the absorption by water in many cases. Furthermore, similarly to the above-described laser spectrometry, ammonia gas having a decreased water content as low as negligible must be used as a reference gas but this has been heretofore difficult to obtain.
As described above, conventionally known methods for measuring a water concentration in ammonia have various problems particularly in the case of measuring the water content in a low concentration in ammonia. Thus, more improvements are being demanded.